This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2000-283302, filed Sep. 19, 2000, the entire contents of this application are incorporated herein by reference.
This invention relates to a process for producing silicon carbide fiber reinforced, silicon carbide matrix composites by polymer impregnation and subsequent radiation application. The produced silicon fiber reinforced, silicon carbide matrix composites have high strength and high heat resistance and exhibit a nonbrittle fracture behavior.
Silicon carbide is a heat-resistant and high-strength material and has been commercially available in fiber form and in fabrics such as textiles woven from the fibers. However, on account of its low resistance to mechanical and thermal shocks, the commercial application of silicon carbide has not yet expanded to non-fiber shapes such as the materials of construction of turbine blades and internal-combustion engines. In an experiment conducted by the present inventors, turbine blade samples made of a crystalline silicon carbide shape were subjected to a continuous high-speed rotation test and found to break in about several hundred hours; in addition, the fragments of the broken turbines flew about at high speed to potentially damage the surrounding objects.
To correct this drawback of silicon carbide, its matrix is reinforced with silicon carbide fibers to make a silicon carbide fiber reinforced, silicon carbide matrix composite (hereunder referred to as xe2x80x9csilicon carbide compositexe2x80x9d). Intensive RandD efforts are being made for the silicon carbide composite as a material that maintains the characteristics of the silicon carbide matrix such as high heat resistance and high strength while exhibiting a nonbrittle fracture behavior and which can be processed into larger ceramic shapes.
Three methods are currently used to produce silicon carbide composites, 1) chemical vapor infiltration, 2) molten silicon impregnation, and 3) multiple polymer impregnation (see xe2x80x9cProcessing of Ceramic Matrix Compositesxe2x80x9d, R. R. Naslain, Key Engineering Materials Vols. 164-165 (1999) pp. 3-8).
In chemical vapor infiltration, silicon carbide in fiber or fabric form is preliminarily shaped and subjected to gas-phase reaction between silane gas and a hydrocarbon compound gas to form a matrix between fibers, thereby producing a silicon carbide composite. In this method, the silicon carbide matrix is formed on the surfaces of the reinforcing fibers and between themselves by the high-temperature reaction of the feed gases being borne by a carrier gas, so an unduly long time is required to produce the composite; in addition, a complicated apparatus is necessary and difficulty is involved not only in forming the matrix uniformly throughout the shape but also in producing large composites.
In molten silicon impregnation, carbon particles are filled between silicon carbide fibers which are then immersed in a molten silicon bath so that carbon reacts with silicon to form a silicon carbide composite. The main problem with this method is that carbon and silicon do not undergo stoichiometric reaction but remain unreacted here and there in the matrix, causing defects such as reduced resistance to oxidation and lower strength at elevated temperatures.
In multiple polymer impregnation, silicon carbide fibers are impregnated with a precursor polymer (e.g. polycarbosilane) which becomes a matrix by firing and the precursor polymer is then fired to a ceramic state, thereby forming a silicon carbide composite. This method can produce the ceramic composite more easily than chemical vapor infiltration and it has many other advantages such as the ability to form a uniform microfine structure in the matrix and suppress fracture due, for example, to stress concentration. On the other hand, the silicon-based polymer shrinks as it turns to an inorganic ceramic state upon firing and its volume decreases to about one half the volume of the initial polymer. To deal with this problem, the impregnation and firing cycles must be repeated 7 to 10 times in the usual process of producing the silicon carbide composite by polymer impregnation. Another problem with the multiple polymer impregnation concerns the pyrolysis that is performed in an oxidizing atmosphere to make the silicon-based polymer infusible (through oxygen-mediated crosslinking of polymer molecules) and the oxygen that is eventually incorporated into the silicon carbide composite contributes to a marked drop in its heat resistance.
Each of the three methods for producing silicon carbide composites has the need to treat the surfaces of the reinforcing silicon carbide fibers with boron nitride or carbon. Otherwise, the produced silicon carbide composites undergo brittle fracture (see xe2x80x9cProcessing of Ceramic Matrix Compositesxe2x80x9d, supra, and xe2x80x9cFine Ceramic Fibersxe2x80x9d, Anthony R. Bunsell, Marie-Hxc3xa9lxc3xa8ne Berger and Anthony Kelly, pp. 1-62 in Fine Ceramic Fibers, edited by Anthony R. Bunsell and Marie-Hxc3xa9lxc3xa8ne Berger, Marcel Dekker, Inc., New York, Basel, 1999). This is because the silicon carbide matrix formed by firing binds directly with the silicon carbide fibers and the resulting integral structure breaks under impact (brittle fracture occurs). Then there is no sense in forming the composite by a complex procedure. On the other hand, the treatment with boron nitride intended to prevent brittle fracture is applied to the entire surfaces of the silicon carbide fibers by a chemical gas-phase route and is a very time-consuming and costly process.
The silicon carbide composite also has the potential to be used as the material of construction of the inner walls of nuclear fusion reactors. Since carbon and silicon emit only short-lived radioactive substances upon irradiation with neutrons, the silicon carbide composite is a promising heat-resistant material that emits low radio-activity. To realize this expectation, the concentrations of impurities must be lowered and, in particular, the contents of nitrogen and metallic elements have to be made considerably lower than the heretofore tolerable levels. However, contamination by nitrogen and metallic atoms has been unavoidable in the multiple polymer impregnation and molten silicon impregnation processes.
As described above, no method has been established to date that is capable of producing silicon carbide composites that have high heat resistance, high strength and high purity while exhibiting a nonbrittle fracture behavior.
An object, therefore, of the present invention is to provide a process by which silicon carbide composites that have high strength, high heat resistance and high purity while exhibiting a nonbrittle fracture behavior can conveniently be produced by blending polycarbosilane and polyvinylsilane as two silicon-based polymers in a specified ratio, impregnating the resulting polymer blend in silicon carbide fibers to prepare a preceramic molding body, exposing the preceramic molding body to radiation and firing the irradiated preceramic molding body.